The invention relates to motor control anti-windup and voltage saturation design for electric power steering (EPS).
Electric power steering systems require the electric motor providing steering assist to be operated using a method of torque control. When using a Permanent Magnet Synchronous Machine (PMSM), Field Oriented Control (FOC) is utilized. This allows an alternating current (AC) three-phase motor voltage and associated current signals to be transformed into a synchronously rotating reference frame (i.e. d/q axis reference frame). In the d/q axis reference frame, the motor voltages and currents become direct current (DC) quantities. The FOC torque control technique may be implemented either using feedforward methods of control or a closed-loop current feedback control.
Closed-loop current control of electric motors in electric power steering systems has demanding requirements outside of the control system's capability to track the desired assist torque command (e.g. motor torque command). Among these, consistency of performance throughout the operating range of the control system is required, including operation throughout the motor velocity range and operation approaching the supply voltage limit.
Unlike high voltage power applications utilizing PMSMs, the supply voltage available for the control system from the vehicle has limits, and the motor used in vehicle applications is typically sized to deliver steady state power requirements. For closed loop current feedback motor control architectures, the control system is designed to track current step commands with near-zero steady state error. Two integrators, one for each current loop, may be utilized for this purpose. The current controller applies a suitable transformation on the reference and measured currents, in order to obtain voltage commands, which are then applied to the motor via a voltage source inverter (VSI).
Some current control systems are designed as quasi-linear feedback architectures expected to satisfy a given set of linear system performance metrics. However, non-linear constraints associated with the actuator can cause the linear feedback control loop to destabilize when the actuator limits are reached, resulting in degraded overall system performance. This degraded performance can manifest itself both in terms of tracking behavior, torque ripple and audible noise. One such non-linearity in the motor control system is that the total voltage applied to the motor is limited by the amount of voltage available at the input of the VSI. If the linear control loop computes voltage commands that result in an overall voltage command magnitude that is beyond the output of the battery, the voltage commands may need to be limited.
An issue that arises due to the presence of a plant input saturation nonlinearity is that when the controller computes voltage commands (transient or steady-state) beyond the voltage limit of the system, the voltage commands are limited to the maximum available battery voltage. States (or element of the controller that has memory), such as that of an integrator, are incorrect, because they do not conform to the non-limited or pre-limited voltage commands originally computed. If the saturated condition lasts for a period of time, the states of the controller may become highly incorrect. When the system returns to the linear operating range, the states can return to correct values after a certain amount of time, which is dependent on how long the saturation condition lasted. This situation, referred to as controller windup, can produce poor overall control system performance and instability.
Some anti-windup schemes are focused on single-input single-output (SISO) systems, especially PID type controllers. There have been recent advancements in design methodologies for multi-input multi-output (MIMO) AW systems with plant input nonlinearities. However, these techniques are primarily focused on MIMO systems that have independent saturation limits on each control signal. In a PMSM current control system, the nonlinearity is a radial or circular voltage limit, in which both the control signals saturate together.